One important feature on most electronic copiers and duplicators is the decrease or increase of the size of the copy of an original by a non-integer factor. If the original image is available, there are numerous ways to accomplish this. However, frequently, the image has been scanned at a previous time, and the image is stored in memory. The image can be digitally stored as gray scale, typically six or eight bits per pixel, but this requires a large memory. The more common alternative is to halftone the image, resulting in a storage requirement of one bit per pixel. Later, the operator may want to enlarge or reduce the image size, preferably without changing the halftone pitch, which is optimized to the characteristics of the printer. The two most common methods are to delete or add either bits or dots according to the "nearest neighbor" algorithm. Each has its disadvantages.
The deletion of bits, or pixels, from the image results in Moire patterns. These are particularly noticeable, and undesirable, in the broad areas of solid color in the image where there is otherwise little change of image density. In addition, the screen pitch is changed by the amount of the factor. On the other hand, the dropping or adding of entire halftone dots maintains the halftone pitch but results in an unacceptable jaggedness of the image edges. What is required is an image enlargement or reduction process that maintains the same pitch, creates smooth edges and does not create Moire patterns.